Methods for protecting a die surface with photocurable materials

ABSTRACT

In a first aspect of the present invention, a method for manufacturing a flip chip package is provided comprising the steps of a) providing a chip having electrically conductive pads on an active surface thereof, b) coating at least a portion the chip with a protectant composition comprising a polymerizable component comprising a thermosetting epoxy resin, at least 50 weight percent of a substantially transparent filler having a coefficient of thermal expansion of less than 10 ppm/° C., a photoinitator, and a solvent carrier, wherein the protectant composition comprises a thixotropic index of less than 1.5, c) masking the coated chip to mask areas where vias through the protectant are desired, d) exposing the masked chip to a light source sufficient to partially crosslink the protectant composition in the unmasked areas, e) removing the uncured portions of the protectant composition thereby creating vias through the protectant composition to the electrically conductive pads on the surface of the chip, f) applying an electrically conductive material to the chip through the vias, wherein the electrically conductive material protrudes from the surface of the protectant composition, and g) heating the chip to a temperature sufficient to reflow the electrically conductive material and thermoset the protectant composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119(e) fromU.S. Provisional Patent Application Ser. No. 61/117,707, filed Nov. 25,2008, entitled “UV CURABLE EPOXY AND METHODS FOR PROTECTING A DIESURFACE AND INTERCONNECTS IN A DIE PACKAGE”, and U.S. Provisional PatentApplication Ser. No. 61/174,147, filed Apr. 30, 2009, entitled “UVCURABLE EPOXY AND METHODS FOR PROTECTING A DIE SURFACE AND INTERCONNECTSIN A DIE PACKAGE”, the disclosures of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention is directed to microelectronic chip assemblies andin particular, methods and materials for applying a photocurableprotectant composition to integrated circuit wafers.

BACKGROUND OF THE INVENTION

Surface mounting of electronic components is well developed in automatedpackage assembly systems. Integrated circuits are made up of devicessuch as transistors and diodes and elements such as resistors andcapacitors linked together by conductive connections to form one or morefunctional circuits. The devices are built on wafers, or sheets ofsilicon with a surface that is subject to a series of fabrication stepsto form a pattern of identical integrated circuits separated from eachother by a repeating rectangular pattern of scribe lines or saw streetsin the surface of the wafer that serve as boundaries between the chip ordie. At a late stage in a fabrication process the singulated die fromthe wafer is bonded to a substrate to form an IC package.

Conventional flip chip technology generally refers to any assembly wherethe active side of the integrated circuit die is attached to a packagesubstrate or printed circuit board (collectively referred to as a PCB).In connection with the use of flip chips, the chip is provided withbumps or balls of solder (hereafter “bumps” or “solder bumps”)positioned in locations on the active side designed to correspond to theinterconnect areas or pads on the surface of the circuit board. The chipis mounted by registering the bumps with the board such that the solderbumps become sandwiched between the pads on the board and the chip. Heatis applied to the assembly to a point at which the solder is caused tomelt, flow, and contact fully the pads on the board (referred to asreflow). Upon cooling, the solder hardens, thereby mounting the flipchip to the board's surface. Conventional underfill materials are usedin several distinct approaches and are applied to a mounted chip toprovide protection of the chip against chemical attack, moisture,radiation, air-borne contaminants, and the like, as well as againstmechanical shock, vibration, and temperature cycling encountered intransit as well as use. A conventional capillary flip chip underfillprocess entails the steps of alignment of chip and circuit board, fluxdispensing, solder reflow, flux cleaning, underfill application,underfill flow and curing.

Underfill materials used in chip packages serve functions to protect thesolder joints that interconnect the chip and package or board fromenvironmental factors such as moisture and contaminants and toredistribute mechanical stresses, which in turn increases devicelifetime. Protection is provided for the chip against contaminants suchas moisture and resulting corrosion of the metal interconnects. Howeverimproper selection of adhesives can result in flip chip package failuresin several modes, such as shrinkage, delamination, hydrolyticinstability, corrosion, and contamination by the underfill.

Chip underfill materials are designed to avoid imparting stress betweenthe adherends as a result of differential coefficients of thermalexpansion between the chip, interconnects, underfill and substrate.Failure modes due to stresses become more prevalent if the substrate isorganic and as device size increases. A chip underfill must provide thefunction of adhering to the substrate, which may or may not be coatedwith solder mask; metal alloy or organic interconnects; and theintegrated circuit die (chip), typically comprised of silicon or otherinorganic species and may or may not be coated with an organicpassivation layer.

In one of two principle ways to package electronic components, thecomponents are soldered to the same side of the board upon which theyare mounted. These devices are said to be “surface-mounted”. Two typesof conventional underfill materials are in practice for use withsurface-mounted devices: capillary flow and “no flow” types. Detaileddescriptions of these technologies can be found in the literature. Forexample, see John H. Lau's book Low Cost Flip Chip Technologies for DCA,WLCSP and PBGA Assemblies, McGraw-Hill, 2000. For both of thesetechnologies, heat is typically used to either cure a liquidthermosetting formulation or laminate a solid film into the assembly.Vacuum is sometimes used to remove air voids from the system. Theunderfill is typically applied on the surface mount (SMT) assembly linefor chip in-package or chip on-board. The use of the traditional flowand no-flow underfills requires several steps on the SMT line, and thisprocess is usually the bottleneck on these microelectronics assemblylines.

Flip chip type electronic packages suffer from sensitivities to impactand thermal stresses. These components typically fail at the electricalinterconnects (such as solder bumps), or the dielectric layers on thesilicon. An underfill is typically employed to secure the solder jointsand protect the silicon die from exposure to extreme stress and/orcorrosive environments.

Uncured liquid underfills are typically dispensed after the electricalinterconnections are made, and cured to provide a mechanical bondbetween the die and board. The underfills also mechanically brace thesolder and die, transferring stress away from the areas most prone tofailure.

Another type of underfill is the wafer applied solder brace coating,which like an underfill, braces the solder and reinforces the die. Butunlike liquid dispensed underfills, the wafer applied solder braces donot form a bond between the die and board. Wafer applied solder bracesalso are applied prior to bonding of the die to the board, typically atthe wafer level (prior to dicing).

There are several methods of coating a wafer level solder bracingmaterial. One method is to coat the wafer uniformly with the bracingmaterial, then pattern holes into the coating by laser drilling or UVexposure. The subsequent holes are then filled with solder to make theelectrical interconnect. This method has difficulties with alignment;the opaque coating material covers the entire surface of the wafer, andalignment of the drilling or UV exposure tools is made difficult.Further issues are related to cleaning the residue in the patternedholes for efficient soldering.

Additionally, passivation films and other semiconductor die coatings arecommonly used as a barrier to physical damage and environmentalcontaminants. In the manufacture of semiconductor devices, the entiretop surface of the wafer is often coated with a passivation filmfollowing the formation of the final metal layer. The passivation filmis an insulating protective layer that minimizes damage to the diesduring assembly and packaging. Passivation film may comprise inorganiccompounds such as phosphosilicate glass and silicon nitride or organiccompounds such as polyimides. The polyimide film is spun on to the waferas a liquid polyamic-acid precursor. During high temperature curing orwith the aid of a photoinitiator, the polyamic acid undergoes a chemicalchange called imidization that causes it to become the solid polyimidefilm.

Further, dielectric layers are provided to isolate the variouselectrical components on the die and provide an electrically insulatingfunction. Vias are created through the dielectric layer to enableelectrical interconnects to be established through the various layers ofthe chip package through standard photo-resist processing.

SUMMARY OF THE INVENTION

In a first aspect of the present invention, a method for manufacturing aflip chip package is provided comprising the steps of a) providing achip having electrically conductive pads on an active surface thereof,b) coating at least a portion the chip with a protectant compositioncomprising a polymerizable component comprising a thermosetting epoxyresin, at least 50 weight percent of a substantially transparent fillerhaving a coefficient of thermal expansion of less than 10 ppm/° C., aphotoinitator, and a solvent carrier, wherein the protectant compositioncomprises a thixotropic index of less than 1.5, c) masking the coatedchip to mask areas where vias through the protectant are desired, d)exposing the masked chip to a light source sufficient to partiallycrosslink the protectant composition in the unmasked areas, e) removingthe uncured portions of the protectant composition thereby creating viasthrough the protectant composition to the electrically conductive padson the surface of the chip, f) applying an electrically conductivematerial to the chip through the vias, wherein the electricallyconductive material protrudes from the surface of the protectantcomposition, and g) heating the chip to a temperature sufficient toreflow the electrically conductive material and thermoset the protectantcomposition.

In another aspect of the present invention, the method further includesa temporary substrate on which the chip is positioned. In this methodthe chip is provided on a temporary substrate which is larger in areathan the chip; during step b) at least a portion of the temporarysubstrate adjacent to the chip is coated with the protectantcomposition; during steps c) and d) the coated portions of the temporarysubstrate are masked and exposed, and in step e) any uncured portions ofthe protectant on the temporary substrate are removed to provide viasthrough the protectant coating to the temporary substrate. In a furtherembodiment of this aspect of the present invention, an additional stepof applying an electrically conductive material to the vias of the chipand the vias of the temporary substrate is added. In still anotherembodiment of the present invention, the electrically conductivematerial applied to the vias of the chip comprises solder and theelectrically conductive material applied to the vias of the temporarysubstrate comprises an electrically conductive paste.

In a further embodiment of the present invention, the electricallyconductive material comprises solder balls. In an additional embodimentof the present invention, the substantially transparent filler comprisesa coefficient of thermal expansion of less than 2 ppm/° C. In a furtherembodiment of the present invention, during step g) the temperature isat least 120° C. And in yet another embodiment of the present invention,a flux composition is applied in the vias prior to step f). And in a yetfurther embodiment of the present invention, step e) comprises exposingthe chip to a development solution by an impinging spray.

In another embodiment of the present invention, step b) is accomplishedthrough at least one of spin coating, screen printing, or stencilprinting. In a further embodiment of the present invention, the chip isprovided as a plurality of chips comprising a wafer. In a furtherembodiment of the present invention, the method further comprises thestep of dicing the wafer to create individual dies. In anotherembodiment of the present invention, during the step of dicing the waferis aligned through visual means through the protectant composition.

In one embodiment of the present invention, the coefficient of thermalexpansion of the thermoset protectant composition is less than 20 ppm/°C. In another embodiment of the present invention, prior to step c) thematerial is b-staged to form a solid composition by removing thesolvent. In a further embodiment of the present invention, the b-stagingis accomplished by heating the coated chip to a temperature notexceeding 120° C.

In an additional embodiment of the present invention, the substantiallytransparent filler comprises fused silica. In a preferred embodiment ofthe present invention, the filler comprises an average particle size ofgreater than 0.40 microns. In a most preferred embodiment of the presentinvention, less than 5 weight percent of the filler particles have aparticle size of less than 0.10 microns.

In yet another embodiment of the present invention, the polymerizablecomponent of the protectant composition comprises at least 98 percent ofan epoxy-based material. In a further embodiment of the presentinvention, the polymerizable component of the protectant compositionconsists essentially of an epoxy-based material.

In a preferred embodiment of the present invention, the filler has arefractive index, and the epoxy is selected to have a refractive indexof within 10 percent of the refractive index of the filler. In anotherembodiment of the present invention, the filler is present in an amountfrom 50 weight percent to about 90 weight percent based on the totalweight of the protectant composition. In a more preferred embodiment ofthe present invention, the filler is present in an amount from 65 weightpercent to about 75 weight percent based on the total weight of theprotectant composition. In a most preferred embodiment of the presentinvention, the filler is present at about 70 weight percent based on thetotal weight of the protectant composition.

In another embodiment of the present invention, the epoxy resin ispresent in an amount from 10 weight percent to 50 weight percent basedon the total weight of the protectant composition. In a furtherembodiment of the present invention, the epoxy resin is present in anamount from 25 weight percent to 35 weight percent based on the totalweight of the protectant composition. In a still further embodiment ofthe present invention, the epoxy resin is present at about 10 weightpercent based on the total weight of the protectant composition.

In one embodiment of the present invention, the solvent is present atabout 15 weight percent based on the total weight of the protectantcomposition. And in another embodiment of the present invention, thephotoinitiator is present in an amount from 0.1 to 2.5 weight percentbased on the total weight of the protectant composition.

In a preferred embodiment of the present invention, the protectantcomposition is substantially absent polyimides or polyimide precursors,and in a most preferred embodiment of the present invention, theprotectant composition is completely free from polyimides or polyimideprecursors. Additionally, in another preferred embodiment of the presentinvention, the protectant composition is substantially absent acrylates.

Thus, there has been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thatfollows may be better understood and in order that the presentcontribution to the art may be better appreciated. There are, obviously,additional features of the invention that will be described hereinafterand which will form the subject matter of the claims appended hereto. Inthis respect, before explaining several embodiments of the invention indetail, it is to be understood that the invention is not limited in itsapplication to the details and construction and to the arrangement ofthe components set forth in the following description. The invention iscapable of other embodiments and of being practiced and carried out invarious ways.

It is also to be understood that the phraseology and terminology hereinare for the purposes of description and should not be regarded aslimiting in any respect. Those skilled in the art will appreciate theconcepts upon which this disclosure is based and that it may readily beutilized as the basis for designating other structures, methods andsystems for carrying out the several purposes of this development. It isimportant that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect of the present invention, a photo-curable epoxymicroelectronics protectant composition is provided which can be appliedto the surface of a wafer as a stress buffer and electrically isolativematerial. The single material can take the place of multipledielectrics, such as such as photo-definable polyimides, moldingcompounds, part-underfills.

In one embodiment of the present invention, the protectant compositioncomprises a photocurable epoxy resin, a filler that is substantiallytransparent, a solvent carrier and a photoinitiator, wherein theprotectant composition is substantially transparent.

The protectant composition materials employed in the present inventioncomprise those which are photo-definable as well as thermally curable.As such, the material can be at least partially hardened or gelled whenexposed to UV or other photo-radiation, then are more fully cured orhardened when heated during the reflow process.

In one embodiment of the present invention, the protectant compositioncomprises an optically and UV transparent, or substantially transparent,material. It is desirable to have a degree of UV transparency so as toensure adequate cure by the photoinitiator during the masking/viacreating step, and optical transparency is desirable to aid in visualalignment for wafer dicing and mask alignments for UV exposure andsolder ball drop. For the purposes of this invention, “substantiallytransparent” is meant to include a protectant material that will curewhen exposed to photo-illumination and features on a substrate can beviewed through the material for identification and alignment purposes,at an applied material thickness of 50 μm.

Selection of the components of the protectant composition is driven bythe need for transparency, as well as a desire to match the coefficientof thermal expansion of surrounding materials at normal operatingtemperatures. Epoxy resins generally have a coefficient of thermalexpansion in the range of 50-80 ppm/° C., however the coefficient ofthermal expansion of the silicon die is about 2.8 ppm/° C. As such, itis preferable to employ a filler which has a very low coefficient ofthermal expansion so as to reduce the coefficient of thermal expansionof the protectant composition as much as possible or to match thecoefficient of thermal expansion of the interconnect. In a preferredembodiment of the present invention, the protectant compositioncomprises a coefficient of thermal expansion of less than 20 ppm/° C. ator around room temperature.

In another preferred embodiment of the present invention, the protectantcomposition has a viscosity and thixotropic properties which allow forspin coating or printing of the material onto a wafer. The desirablethixotropic properties are best measured by the thixotropic index,wherein the thixotropic index is a ratio of viscosity at a shear rate of1/s to the viscosity at a shear rate of 10/s at room temperature (about20° C.). In a preferred embodiment of the present invention, thethixotropic index is less than 1.5, and in a most preferred embodimentof the present invention, the thixotropic index is less than 1.3.

The substantially transparent filler can be organic, inorganic or amixture thereof. The fillers for use in the present invention comprisethose which are UV and optically transparent so as not to interfere withthe UV curatives and allow for alignment of masks for photo-definitionof the protectant. Additionally, the fillers suitable for use in thepresent invention are thermally conductive, but not electricallyconductive, i.e. low dielectric. Additionally, the preferred fillers ofthe present invention have a low coefficient of thermal expansion,preferably as close to zero or negative as possible. For this reason,fillers of crystalline structure are particularly preferred. In a mostpreferred embodiment of the present invention, the coefficient ofthermal expansion of the filler is less than 10 ppm/° C., morepreferably less than 5 ppm/° C., and most preferably less than 2 ppm/°C.

Most preferred fillers comprise those which are UV transparent such thatif a film of 100 μm thickness is prepared by photocuring a photocurablecomposition containing 70 percent by weight of the filler and a broadband exposure wavelength of 500 nm to 200 nm UV light is irradiated ontothe film, at least 40 percent of the light is transmitted through thefilm, relative to the amount of light transmitted through a control filmof the same thickness containing no filler.

In one embodiment of the present invention, the filler comprises silicondioxide. Silicon dioxide is preferred in applications wherenon-electrically conductive filler is desirable. In another embodimentof the present invention, suitable fillers include, diamond, quartz,silicon carbide, substantially transparent metal oxides, zirconiumoxides, and the like.

In one embodiment of the present invention, the filler comprises anaverage particle size from about 0.40 microns to about 30 microns. In amore preferred embodiment of the present invention, the fuller comprisesan average particles size of about 0.6 microns to 5.0 microns. Inanother embodiment of the present invention, it is preferred to avoidvery small filler particles due to negative effects on thixotropy and assuch, less than 5 weight percent of the filler particles should be lessthan 0.10 microns.

Preferably, the filler is present in an amount from 50 weight percent toabout 90 weight percent based on the total weight of the protectantcomposition, and more preferably the filler is present in an amount from65 weight percent to about 75 weight percent based on the total weightof the protectant composition. In a most preferred embodiment of thepresent invention, the filler is present at about 70 weight percentbased on the total weight of the protectant composition.

Photocurable epoxies are well known polymeric materials characterized bythe presence of oxirane functionality, and which are curable through acationic induced polymerization mechanism. Suitable photocurable epoxiesinclude cycloaliphatic epoxy monomers, oligomers, or combinationsthereof.

In one embodiment of the present invention, the photocurable epoxy maycomprise monofunctional and multifunctional glycidyl ethers ofBisphenol-A and Bisphenol-F, aliphatic and aromatic epoxies, saturatedand unsaturated epoxies, cycloaliphatic epoxy resins and combinations ofthose. Another suitable epoxy resin is epoxy novolac resin, which isprepared by the reaction of phenolic resin and epichlorohydrin. Apreferred epoxy novolac resin is poly(phenyl glycidylether)-co-formaldehyde. Other suitable epoxy resins are biphenyl epoxyresin, commonly prepared by the reaction of biphenyl resin andepichlorohydrin; dicyclopentadiene-phenol epoxy resin; naphthaleneresins; epoxy functional butadiene acrylonitrile copolymers; epoxyfunctional polydimethyl siloxane; and mixtures of the above.Non-glycidyl ether epoxides may also be used. Suitable examples include3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, whichcontains two epoxide groups that are part of the ring structures and anester linkage; vinylcyclohexene dioxide, which contains two epoxidegroups and one of which is part of the ring structure;3,4-epoxy-6-methyl cyclohexyl methyl-3,4-epoxycyclohexane carboxylate;and dicyclopentadiene dioxide.

The epoxy component of the protectant composition is selected forsolvent compatibility, melting/softening temperature, adhesion tosubstrates, cured modulus, cured CTE, and ionic residue. In mostmicroelectronic applications, adhesion and ionic reside are the mostimportant parameters. Further, the epoxy chosen or use in the presentinvention must be substantially transparent to UV and visible light.

In a preferred embodiment of the present invention, the epoxy resin isselected to have a refractive index when cured which matches therefractive index of the filler to allow greater transparency of thecomposition. In one preferred embodiment of the present invention,silica with a refractive index of about 1.46 is selected as the filler.In this embodiment, the refractive index of the resin is preferablybetween 1.4 to 1.7 and more preferably less than 1.5.

As such, in a preferred embodiment of the present invention, therefractive index of the epoxy and refractive index of the filler arechosen to be with in 10 percent of each other, and most preferablywithin 7 percent and even more preferably within 5 percent or less.

In one embodiment of the present invention, the epoxy resin is presentin an amount from 10 to 50 weight percent, preferably 25 to 35 weightpercent based on the total weight of the uncured material, and morepreferably the epoxy resin is present at about 10 weight percent basedon the total weight of the uncured material.

In an embodiment of the present invention, it is preferred not to have asignificant quantity of other polymerizable components in the protectantcomposition other than the epoxy and photoinitiator components. As such,the polymerizable component of the protectant composition comprises atleast 98 percent of an epoxy material. In a more preferred embodiment ofthe present invention, the protectant composition is substantiallyabsent, or alternatively, completely free of polyimides or polyimideprecursors. In another embodiment of the present invention, theprotectant composition is substantially absent, or alternativelycompletely free, of acrylates.

In one embodiment of the present invention, the photoinitiator is chosento partially crosslink the epoxy resin at a desired wavelength. When theprotectant materials polymerizes under the influence of UV or otherphoto-radiation, the material is converted from a solid or stiff gel orpaste at ambient temperatures to an at least partially crosslinked solidincapable of resolvation, preferably having a tack free surface. The atleast partially crosslinked solid remains as a thermoplastic, meaning,it remains in a heat-liquefiable state until fully thermally cured.

These photoinitators release reactive cations when exposed to aparticular wavelength of light. Generally, such photoinitators compriseorganic onium salts, preferably containing sulfur or iodine as thecentral atom of the cation. In one embodiment of the present invention,the photoinitiator comprises at least one of diaryliodonium salts,triarylsulfonium salts, and mixtures thereof. One suitable photoinitatorcomprises triarylsulfonium hexafluorophosphate. Preferredphotoinitiators comprise those with low residual ionics and aresubstantially absent toxicity issues.

In one embodiment of the present invention, the photoinitiator ispresent in an amount from 0.1 to 2.5 weight percent based on the totalweight of the protectant composition. In a more preferred embodiment ofthe present invention, the photoinitiator is present in an amount from0.5 to 1.5 weight percent based on the total weight of the protectantcomposition.

Any source of actinic light dose that does not raise the coatingtemperature above about 120° C. can be used in carrying out thephotocuring solidification of the protectant composition to the solidliquefiable gel state. Ultraviolet light is most readily employed, aswell as other forms, such as Type RS Sunlamps, carbon arc lamps, xenonarc lamps, mercury vapor lamps, tungsten halide lamps and the like. Theradiation energy may emanate from a point source or in the form ofparallel rays. Divergent beams, are however, also operable as a sourceof actinic light. A UV dosage in a range of 100 to 2400 mJ/cm² iseffective to provide the necessary depth of cure. In a preferredembodiment of the present invention, the protectant compositionphotocures to a tack-free surface. Curing periods may be adjusted toproper choice of ultraviolet source, photocuring componentconcentration, and the like.

In an embodiment of the present invention, suitable solvents includethose which are suitably volatile so as to evaporate at normalprocessing temperatures of about 100° C. to 250° C. Suitable solventsmay include one or more organic solvents, such as 1-methoxy-2-propanol,methoxy propanol acetate, butyl acetate, methoxy-ethyl ether, methanol,ethanol, isopropanol, ethylene glycol, methyl-ethyl ketone,cyclo-hexanone, benzene, toluene, xylene, and cellosolves such asethylcellosolve, ethyl acetate, cellosolve acetate, butyl cellosolveacetate, carbitol acetate, and butyl carbitol acetate, and combinationsof thereof.

In a preferred embodiment of the present invention, the solventcomprises the ketone-type solvents such as methyl isobutyl ketone(MIBK), acetone, and methyl ethyl ketone (MEK).

In a preferred embodiment of the present invention, after application ofthe protectant composition to a wafer or die, the solvent is extractedto form a semi-solid protectant composition, also known as “b-staging”.In one embodiment of the present invention, substantially all of thesolvent is removed during the b-staging however trace amounts may stillremain in the protectant composition. Most importantly during this stepis that the liquid or paste applied protectant composition is driedsufficiently to form a solid composition.

In one embodiment of the present invention, the solvent is present in anamount from 10 weight percent to 20 weight percent based on the totalweight of the uncured material, and preferably in an amount of about 15weight percent based on the total weight of the protectant composition.

In an additional embodiment of the present invention, other componentssuch as secondary fillers, wetting agents, sensitizer, thixotropicagents, adhesion promoters, and other additives may optionally bepresent.

In a further aspect of the present invention, the curable material issubstantially absent polyimides or polyimide precursors. In a stillfurther aspect of the present invention, the curable material iscompletely free from polyimides or polyimide precursors.

The protectant compositions of the various embodiments of the presentinvention has particular utility in wafer-level chip fabrication.Additionally, the materials and methods of the present invention haveutility in many flip chip manufacturing processes a few of which arepreferred and will be outlined herein. The photo-curable materialachieves these benefits by having low cure stress and havingcustomizable mechanical properties, such as CTE, modulus, Tg, andadhesion.

Unlike traditional polyimides (PI), the protectant composition of thepresent invention may be applied as a thick coating, preferably between20 μm and 200 μm and most preferably between 25 μm and 60 μm. Further,this material can take the place of the polyimide layer and provideunderfill properties as well. In one embodiment of the presentinvention, multiple layers can be used, with metal trace applicationbetween, which allows this material to be used as a redistributionlayer. This technique makes the material particularly useful forembedding high density electronic traces into molding compounds or 3Dpackages.

When combined with known soldering techniques, this material can alsotake the place of stress buffer coatings, such as wafer appliedunderfills. Vias can be created for solder bumping on the surface, inthe middle, or underneath the new coating material. These vias can be ofany shape or size as desirable in a particular application.

The protectant composition can also be combined with existing dielectrictechnologies, such as polyimides to give symbiotic benefits. One examplewould be a Chip Scale Package (CSP) using a PI redistribution layer(RDL), coupled to the protectant composition as the final passivationlayer. Thus, the protectant composition's main purpose is to protect thecopper traces on the surface of the PI. If a solder bump site is createdon the PI RDL surface, than the protectant composition can also fill amechanical function, as an in-part underfill. The inverse laminate mayalso be preformed, where the PI is used as a protective layer for themetal traced protectant composition dielectric. A further design enabledby this material would be a multi-layer laminate which interfaces thesolder interconnects at different points. One example would be where thesolder interconnects with the non-PI defined die pads and the epoxyencapsulates the solder.

In a preferred embodiment of the present invention, a method formanufacturing a flip chip package is provided. First, a chip or die isprovided having electrically conductive pads on an active surfacethereof. Dies are built on wafers and as such, the die may be singulatedand processed individually, or alternately, remain affixed to the waferfor wafer-level processing. In a preferred embodiment of the presentinvention, the methods are carried out at the wafer level, prior tosingulation of the die. However, one skilled in the art will recognizethat the materials and methods of the present invention may be employedat the wafer level, on an individual die, or on a plurality of diesimultaneously.

In one embodiment of the present invention, the photocurable protectantcomposition is applied to the wafer by at least one of spin coating,printing, spraying, stencil applied, or adhesive film applicationtechniques as are known in the art. In a preferred embodiment of thepresent invention, the protectant composition is spin coated as aviscous liquid to coat wafer to a thickness of greater than 20 μm andpreferably to a predetermined thickness which is less than that of thesolder balls to be applied.

In another embodiment of the present invention, the protectantcomposition is screen or stencil printed onto the die so as toselectively apply the composition to predetermined portions of the diesurface. For example, certain areas can be left free of the compositionsuch as saw streets, alignment marks, electrical interconnect pads,areas pre-bumped, and edge exclusion zones. In this manner the amount ofprotectant composition applied to the die can be reduced as can the needfor subsequent removal of unnecessary protectant composition.

In another embodiment of the present invention, the solvent is removedfrom the protectant composition to solidify the composition on thewafer. The solvent may be removed through air drying at ambienttemperature, and/or under vacuum, however in a preferred embodiment ofthe present invention, the coated wafer is heated to drive off thesolvent. Care must be taken not to heat the material so much so as toinitiate thermal cure of the epoxy, however if the temperature is keptbelow 120° C., preferably below 100° C., excessive hardening of theepoxy can be avoided. After eradication of the solvent the formulationis said to be b-staged and can be stored for extended periods of time,if desired, before further processing.

In another embodiment of the present invention, the b-staged protectantmaterial is masked to provide shadowing in areas where vias, orpathways, through the protectant are desired. The wafer is then exposedto a light source sufficient to partially crosslink the protectantcomposition in the unmasked areas through the action of thephotoinitiator. Selection of the type and quantity of photoinitiatorwill tailor the cure profile as is suitable for a particular operation.The exposed portions of the protectant material are at least partiallycrosslinked, where the masked portions are left uncured and can beremoved.

Removal of the uncured portions of the protectant material may beaccomplished through several methods. In one embodiment of the presentinvention, the material is exposed to a development solution, or solventwash, which is capable of washing the uncured material away therebyexposing the vias. The development solution may be sprayed on or,alternately, the wafer maybe immersed in the solution. In anotherembodiment of the present invention, ultrasonic vibration is employed toremove the uncured portions of the protectant composition. In apreferred embodiment of the present invention, the vias are createdthrough the entire thickness of the protectant composition to expose theelectrically conductive pads on the active surface of the chip.

Once vias are created, solder balls, or other electrically conductivematerial, are applied in the vias through methods which are known in theart. In a preferred embodiment of the present invention, a flux materialis placed on the electrically conductive pads prior to the applicationof solder. The solder balls are then positioned in the vias, preferablysuch that at least a portion of the solder balls extends above thesurface of the protectant composition. With the solder balls in place,the wafer is heated to a temperature sufficient to reflow the solder andcomplete the cure of the protectant composition.

In order to provide adequate reflow of the photo-cured solid, noadvancement of the thermal cure system in the photo-cured solid coatingon the wafer or dice takes place until the solder reflow step. In oneembodiment of the present invention, a latent thermal accelerator as isknow in the art may optionally be employed to regulate cure of epoxy.The thermal cure onset minimum temperature is predetermined by theselection of the epoxy component, and preferably occur after the onsetof solder reflow, at a temperature greater than or equal to 120° C.Preferably, the minimum protectant composition thermal cure onset is inthe range of 120° C. to 225° C. Temperatures of onset of thermal curingshould not be more than about 280° C. The onset of thermal cure shouldnot be too near the peak reflow temperature which is typically at ornear 250° C. for eutectic solder and 300° C. for lead free solder. Atypical solder reflow time takes 3 to 4 minutes, and the protectantcomposition is typically exposed to the peak temperature for less than30 seconds. Thermal cure initiated at temperatures below 120° C. leadsto inadequate protectant composition liquefaction and flow.

In another embodiment of the present invention, the wafer may be furtherprocessed to singulate the die/chips, which may then be mounted on asubstrate. The protectant material is of particular utility in thedicing process because it is transparent. This allows for visualconfirmation of proper alignment of the wafer/dies before the wafer iscut, thereby preventing cutting errors and possible damage to the die.

In an alternate aspect of the present invention, a method formanufacturing a flip chip package is provided comprising applying solderballs to a die, which may or may not be part of an uncut wafer, coatingthe die/wafer with a photocurable protectant composition, masking thecoated wafer to expose the area where solder balls are not present,exposing the masked wafer to a UV light source to cure the protectantcomposition in the exposed areas, and developing the exposed coatedwafer to remove any uncured portions of the coating and expose thesolder balls.

The benefit of this process is that alignment of the mask is facilitatedas the solder balls already protrude through the coating. Furthermore,once exposed, the amount of material cleaned is much less, and there isless chance for inhibiting residue.

EXAMPLES

In a first exemplary embodiment of the present invention, a photocurableprotectant composition is provided comprising Formula 1.

Formula 1 Component Weight (g) Epoxy Resin 62.01 Silica (25 μm max)188.3 Wetting Agent 1.21 Triarylsulfonium 1.50 hexafluoroantimonate *Dibutylacetate (solvent) 35.0 Total 288.02 g * (50% in glycol acetate)

The protectant composition of Formula 1 was b-staged to remove solvent,then exposed to UV radiation to photocure the material, followed by abake cycle intended to mimic reflow oven conditions and complete thethermosetting process of the material. Physical characteristics werethen measured as detailed on the table below:

Property Measurement Tg 95° C. CTE (below Tg) 13 ppm/° C. CTE (above Tg)51 ppm/° C. Modulus (−40° C.) 16.6 Gpa Modulus (100° C.) 12.0 GpaTensile modulus (25° C.)  3.9 Gpa Tensile strength   23 Mpa Elongation0.90 percent Die Shear Adhesion >1600 psi

Although the present invention has been described with reference toparticular embodiments, it should be recognized that these embodimentsare merely illustrative of the principles of the present invention.Those of ordinary skill in the art will appreciate that thecompositions, apparatus and methods of the present invention may beconstructed and implemented in other ways and embodiments. Accordingly,the description herein should not be read as limiting the presentinvention, as other embodiments also fall within the scope of thepresent invention as defined by the appended claims.

1. A method for manufacturing a flip chip package comprising: a) providing a chip having electrically conductive pads on an active surface thereof; b) coating at least a portion the chip with a protectant composition comprising a polymerizable component comprising a thermosetting epoxy resin, at least 50 weight percent of a substantially transparent filler having a coefficient of thermal expansion of less than 10 ppm/° C., a photoinitator, and a solvent carrier, wherein the protectant composition comprises a thixotropic index of less than 1.5; c) masking the coated chip to mask areas where vias through the protectant are desired; d) exposing the masked chip to a light source sufficient to partially crosslink the protectant composition in the unmasked areas; e) removing the uncured portions of the protectant composition thereby creating vias through the protectant composition to the electrically conductive pads on the surface of the chip; f) applying an electrically conductive material to the chip through the vias, wherein the electrically conductive material protrudes from the surface of the protectant composition; g) heating the chip to a temperature sufficient to reflow the electrically conductive material and thermoset the protectant composition.
 2. The method of claim 1, wherein in connection with step a) the chip is provided on a temporary substrate which is larger in area than the chip; during step b) at least a portion of the temporary substrate adjacent to the chip is coated with the protectant composition; during steps c) and d) the coated portions of the temporary substrate are masked and exposed, and in step e) any uncured portions of the protectant on the temporary substrate are removed to provide vias through the protectant coating to the temporary substrate.
 3. The method of claim 2, further comprising the step of applying an electrically conductive material to the vias of the chip and the vias of the temporary substrate.
 4. The method of claim 3, wherein the electrically conductive material applied to the vias of the chip comprises solder and the electrically conductive material applied to the vias of the temporary substrate comprises an electrically conductive paste.
 5. The method of claim 1, wherein the electrically conductive material comprises solder balls.
 6. The method of claim 1, wherein the substantially transparent filler comprises a coefficient of thermal expansion of less than 2 ppm/° C.
 7. The method of claim 1, wherein during step g) the temperature is at least 120° C.
 8. The method of claim 1, wherein a flux composition is applied in the vias prior to step f).
 9. The method of claim 1, wherein step e) comprises exposing the chip to a development solution by an impinging spray.
 10. The method of claim 1, wherein step b) is accomplished through at least one of spin coating, screen printing, or stencil printing.
 11. The method of claim 1 wherein the chip is provided as a plurality of chips comprising a wafer.
 12. The method of claim 11, further comprising the step of dicing the wafer to create individual dies.
 13. The method of claim 12, wherein during the step of dicing the wafer is aligned through visual means through the protectant composition.
 14. The method of claim 1 wherein the coefficient of thermal expansion of the thermoset protectant composition is less than 20 ppm/° C.
 15. The method of claim 1, wherein prior to step c) the material is b-staged to form a solid composition by removing the solvent.
 16. The method of claim 15, wherein the b-staging is accomplished by heating the coated chip to a temperature not exceeding 120° C.
 17. The method of claim 1, wherein the substantially transparent filler comprises fused silica.
 18. The method of claim 1, wherein the filler comprises an average particle size of greater than 0.40 microns.
 19. The method of claim 1, wherein less than 5 weight percent of the filler particles have a particle size of less than 0.10 microns.
 20. The method of claim 1, wherein the polymerizable component of the protectant composition comprises at least 98 percent of an epoxy-based material.
 21. The method of claim 1, wherein the polymerizable component of the protectant composition consists essentially of an epoxy-based material.
 22. The method of claim 1, wherein the filler has a refractive index, and the epoxy is selected to have a refractive index of within 10 percent of the refractive index of the filler.
 23. The method of claim 1, wherein the filler is present in an amount from 50 weight percent to about 90 weight percent based on the total weight of the protectant composition.
 24. The method of claim 1, wherein the filler is present in an amount from 65 weight percent to about 75 weight percent based on the total weight of the protectant composition.
 25. The method of claim 1, wherein the filler is present at about 70 weight percent based on the total weight of the protectant composition.
 26. The method of claim 1, wherein the epoxy resin is present in an amount from 10 weight percent to 50 weight percent based on the total weight of the protectant composition.
 27. The method of claim 1, wherein the epoxy resin is present in an amount from 25 weight percent to 35 weight percent based on the total weight of the protectant composition.
 28. The method of claim 1, wherein the epoxy resin is present at about 10 weight percent based on the total weight of the protectant composition.
 29. The method of claim 1, wherein the solvent is present at about 15 weight percent based on the total weight of the protectant composition.
 30. The method of claim 1, wherein the photoinitiator is present in an amount from 0.1 to 2.5 weight percent based on the total weight of the protectant composition.
 31. The method of claim 1, wherein the protectant composition is substantially absent polyimides or polyimide precursors.
 32. The method of claim 1, wherein the protectant composition is completely free from polyimides or polyimide precursors.
 33. The method of claim 1, wherein the protectant composition is substantially absent acrylates. 